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Abstract:

A Ni-based single crystal superalloy which has the following composition:
Co: 0.0 wt % or more to 15.0 wt % or less, Cr: 4.1 to 8.0 wt %, Mo: 2.1
to 4.5 wt %, W: 0.0 to 3.9 wt %, Ta: 4.0 to 10.0 wt %, Al: 4.5 to 6.5 wt
%, Ti: 0.0 to 1.0 wt %, Hf: 0.00 to 0.5 wt %, Nb: 0.0 to 3.0 wt %, Re:
8.1 to 9.9 wt % and Ru: 0.5 to 6.5 wt % with the remainder including Ni
and unavoidable impurities. As a result, the Ni-based single crystal
superalloy which includes more than 8 wt % of Re in the composition ratio
and has excellent specific creep strength and the turbine blade
incorporating the Ni-based single crystal superalloy may be made.

11. A turbine blade which incorporates the Ni-based single crystal
superalloy according to claim 1.

12. A turbine blade which incorporates the Ni-based single crystal
superalloy according to claim 9.

13. A turbine blade which incorporates the Ni-based single crystal
superalloy according to claim 10.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a Ni-based single crystal
superalloy and a turbine blade incorporating the same.

[0002] The present application claims priority from Japanese Patent
Application No. 2009-100903, filed on Apr. 17, 2009, in Japan, the
contents of which are incorporated herein by reference.

BACKGROUND ART

[0003] Turbine blades (stator blades and rotor blades) of aircraft
engines, industrial gas turbines and other systems are often operated in
high-temperature environments for a prolonged time and thus are made of a
Ni-based single crystal superalloy that has an excellent heat resistance.
The Ni-based single crystal superalloy is produced in the following
manner. Al is first added to base Ni to cause Ni3Al to precipitate
for precipitation strengthening. High melting point metals, such as Cr, W
and Ta, are then added to form an alloy which is formed as a single
crystal. The Ni-based single crystal superalloy acquires a metal
structure suitable for strengthening through solution heat treatment at a
predetermined temperature and subsequent aging heat treatment. The
superalloy is called a precipitation hardened alloy which has a crystal
structure with a precipitation phase (i.e., γ' phase) dispersed and
precipitated in a matrix (i.e., γ phase).

[0004] As the Ni-based single crystal superalloy, a first generation
superalloy contains no Re at all, a second generation superalloy contains
about 3 wt % of Re, and a third generation superalloy contains 5 wt % or
more to 6 wt % or less of Re, have been developed. The superalloys of
later generations acquire enhanced creep strength. For example, the first
generation Ni-based single crystal superalloy is CMSX-2 (Cannon-Muskegon
Corporation, refer to Patent Document 1), the second generation Ni-based
single crystal superalloy is CMSX-4 (Cannon Muskegon Corporation, refer
to Patent Document 2) and the third generation Ni-based single crystal
superalloy is CMSX-10 (Cannon Muskegon Corporation, refer to Patent
Document 3).

[0005] The purpose of the third generation Ni-based single crystal
superalloy, CMSX-10, is to enhance creep strength in high-temperature
environments as compared to the second generation Ni-based single crystal
superalloy. The third generation Ni-based single crystal superalloy,
however, has a high composition ratio of Re of 5 wt % or more, which
exceeds the solid solubility limit with respect to the matrix (γ
phase) of Re. The excess Re may combine with other elements in
high-temperature environments and as a result, a so-called TCP
(topologically close packed) phase to may precipitate. A turbine blade
incorporating the third generation Ni-based single crystal superalloy may
acquire an increased amount of the TCP phase when operated for a
prolonged time in high-temperature environments, which may impair creep
strength.

[0006] In order to solve these problems, a Ni-based single crystal
superalloy having higher strength in high-temperature environments has
been developed. In such a superalloy, Ru for controlling the TCP phase is
added and the composition ratios of other component elements are set to
optimal ranges so as to provide the optimal lattice constant of the
matrix (γ phase) and the optimal lattice constant of the
precipitate (γ' phase).

[0007] Namely, a fourth generation Ni-based single crystal superalloy
which contains about 3 wt % of Ru and a fifth generation Ni-based single
crystal superalloy which contains 4 wt % or more of Ru have been
developed. The superalloys of later generations acquire enhanced creep
strength. For example, an exemplary fourth generation Ni-based single
crystal superalloy is TMS-138 (National Institute for Materials Science
(NIMS) and IHI Corporation, refer to Patent Document 4), and an exemplary
fifth generation Ni-based single crystal superalloy is TMS-162 (NIMS and
IHI Corporation, refer to Patent Document 5).

[0008] The fourth and fifth generation Ni-based single crystal
superalloys, however, include a large amount of heavy metal such as W and
Re, in order to enhance the creep strength in high-temperature
environments, and thus have a high specific gravity as compared to the
first and second generation Ni-based single crystal superalloys. As a
result, a turbine blade incorporating the fourth or fifth generation
Ni-based single crystal superalloy is excellent in strength in
high-temperature environments, however, since the weight of the blade is
increased, there are problems that the circumferential speed of the
turbine blade may be decreased and the weight of the aircraft engine and
the industrial gas turbine may be increased.

[0009] In order to solve these problems, a Ni-based single crystal
superalloy which has a low specific gravity as compared to the fourth and
fifth generation Ni-based single crystal superalloys although its creep
strength is high in high-temperature environments has been developed by
specifying a composition range of W to optimal ranges suitable for
keeping excellent creep strength in high-temperature environments and by
specifying a composition range suitable for structural stability, with
reducing an amount of W which has a high specific gravity (refer to
Patent Document 6).

[0010] Furthermore, in recent years, a Ni-based single crystal superalloy
which has a high composition ratio of Re as compared to the
above-described conventional Ni-based single crystal superalloys (the
composition ratio of Re is more than 8 wt % in the concrete) has been
developed (refer to Non-Patent Document 1). This Ni-based single crystal
superalloy is called as a high-rhenium single crystal Ni-base superalloy
in the Non-Patent Document 1 and includes 9 wt % of Re in the composition
ratio as shown in Table 1 of the Non-Patent Document 1.

[0018] In order to develop a Ni-based single crystal superalloy which can
obtain excellent creep strength in high-temperature environments as
compared to the conventional alloys, it is expected that the composition
ratio of Re in the alloy must be increased as described in the Non-Patent
Document 1. Therefore, it is desirable to develop a Ni-based single
crystal superalloy which has a composition ratio of Re higher than the
conventional ratio of 8 wt % in order to improve creep strength of a
turbine blade in high-temperature environments.

[0019] In addition, since this Ni-based single crystal superalloy includes
a large amount of Re which is a heavy metal as compared to the
conventional alloys, it is also desirable to develop a Ni-based single
crystal superalloy having excellent creep strength per unit weight, i.e.,
having excellent specific creep strength, in order to provide a turbine
blade which is lightweight and is operated at higher temperatures.

[0020] In view of these circumstances, an object of the present invention
is to provide a Ni-based single crystal superalloy which includes a large
amount of Re and has excellent specific creep strength and a turbine
blade incorporating the same.

Means for Solving the Problem

[0021] The inventors have made intensive studies and found that a Ni-based
single crystal superalloy which includes Re more than the conventional
alloy and creep strength in high-temperature environments is improved,
and which has a low specific gravity as compared to the fourth and fifth
generation Ni-based single crystal superalloys may be obtained, by (1)
modifying a composition ratio in view of structural stability and control
of the TCP phase together with increasing the composition ratio of Re is
more than 8 wt %, and (2) specifying a composition range suitable for
maintaining excellent creep strength in high-temperature environments
together with including Re which controls the TCP phase and with reducing
an amount of W which has a high specific gravity; and completed the
present invention.

[0023] As described above, according to the present invention, an
excellent creep strength in high-temperature environments can be
maintained without increasing the specific gravity of the Ni-based single
crystal superalloy which includes more than 8 wt % of Re in the
composition ratio. Therefore, the turbine blade incorporating the
Ni-based single crystal superalloy can be made lightweight and can be
operated at higher temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a perspective view of an exemplary turbine blade
incorporating a Ni-based single crystal superalloy according to an
embodiment of the present invention.

[0025] FIG. 2 is a characteristic chart showing a relationship between
content of Re and specific gravity in examples and reference examples
shown in Table 1.

[0026]FIG. 3 is a graph showing a creep rupture time of examples and a
reference example according to Patent Document 1 shown in Table 1.

[0027]FIG. 4 is a graph obtained by simulation showing a relationship
between content of Mo and creep speed in a Ni-based single crystal
superalloy which includes average composition of examples of the present
invention.

[0028] FIG. 5 is a graph obtained by simulation showing a relationship
between content of Mo and precipitation starting time of TCP phase in a
Ni-based single crystal superalloy which includes average composition of
examples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] In the following, a detailed explanation for carrying out a
Ni-based single crystal superalloy and a turbine blade incorporating the
same according to the present invention will be explained in detail with
reference to the drawings.

[0038] In the present invention, in order to obtain a Ni-based single
crystal superalloy which having a low specific gravity, the content of W
in the composition of the Ni-based single crystal superalloy may be 0.0
to 2.9 wt % and more preferably may be 0.0 to 1.9 wt %.

[0039] The metal structure of the above-described Ni-based single crystal
superalloy is a crystal structure with the precipitation phase (γ'
phase) dispersed and precipitated in the matrix (γ phase). The
γ phase consists of an austenite phase and the γ' phase
consists mainly of intermetallic compounds having an ordered structure,
such as Ni3Al. In the Ni-based single crystal superalloy according
to the present invention, the composition ratio of the γ-phase and
the γ'-phase dispersed in the γ-phase may be optimized to
contribute to higher strength of the superalloy to be operated in
high-temperature environments.

[0040] The composition ranges of the components of the Ni-based single
crystal superalloy are controlled based on their characteristics
described below.

[0041] Co is an element that increases the solid solubility limit to the
matrix containing Al, Ta and other elements in high-temperature
environments and causes the fine γ' phase to disperse and
precipitate in heat treatment so as to enhance the high-temperature
strength. If more than 15.0 wt % of Co exists, the composition ratio with
other elements, including Al, Ta, Mo, W, Hf and Cr, becomes unbalanced.
As a result, a harmful phase precipitates to decrease the
high-temperature strength. The content of Co is preferably 0.0 to 15.0 wt
%, and more preferably 4.0 to 9.5 wt %.

[0042] Cr is an element that has excellent oxidation resistance and
improves, altogether with Hf and Al, high-temperature corrosion
resistance of the Ni-based single crystal superalloy. If the content of
Cr is less than 4.1 wt %, it is difficult to provide a desired
high-temperature corrosion resistance. If the content of Cr exceeds 8.0
wt %, precipitation of the γ' phase is inhibited and harmful
phases, such as σ phase and μ phase, may precipitate to decrease
the high-temperature strength. The content of Cr is therefore preferably
4.1 to 8.0 wt %, more preferably 5.1 to 8.0 wt %, and more preferably 5.1
to 6.5 wt %.

[0043] Mo is an element that enhances the high-temperature strength by
dissolving in the phase which becomes the matrix, in the presence of W or
Ta, and also improves high-temperature strength due to precipitation
hardening. If the content of Mo is less than 2.1 wt %, it is difficult to
provide a desired high-temperature strength. If the content of Mo exceeds
4.5 wt %, the high-temperature strength decreases and the
high-temperature corrosion resistance deteriorates. The content of Mo is
therefore preferably 2.1 to 4.5 wt %, more preferably 2.1 to 3.4 wt %,
and more preferably 2.1 to 3.0 wt %.

[0044] W is an element that enhances the high-temperature strength due to
the actions of solution hardening and precipitation hardening in the
presence of Mo or Ta. If the content of W exceeds 3.9 wt %, the
high-temperature corrosion resistance deteriorates. The content of W is
therefore preferably 0.0 to 3.9 wt %. In order to provide a Ni-based
single crystal superalloy having a low specific gravity, the content of W
is preferably 0.0 to 2.9 wt % and more preferably 0.0 to 1.9 wt %. In the
present invention, with a small amount of W or no W at all, excellent
creep strength in high-temperature environments may be kept by
appropriately determining the composition ratio of other component
elements.

[0045] Ta is an element that enhances the high-temperature strength due to
the actions of solution hardening and precipitation hardening in the
presence of Mo or W. Ta also enhances the high-temperature strength by
the precipitation hardening relative to the γ' phase. If the
content of Ta is less than 4.0 wt %, it is difficult to provide desired
high-temperature strength. If the content of Ta exceeds 10.0 wt %, a
harmful phase, such as σ phase and μ phase, may precipitate to
decrease the high-temperature strength. The content of Ta is therefore
preferably 4.0 to 10.0 wt %, more preferably 4.0 to 6.5 wt %, and more
preferably 4.0 to 6.0 wt %.

[0046] Al combines with Ni to form a 60 to 70% (volume percentage) of an
intermetallic compound represented by Ni3Al, which is the fine
γ' phase to be uniformly dispersed and precipitated into the
matrix. That is, Al is an element that enhances the high-temperature
strength altogether with Ni. Furthermore, Al is excellent in oxidation
resistance, which improves, altogether with Cr and Hf, the
high-temperature corrosion resistance of the Ni-based single crystal
superalloy. If the content of Al is less than 4.5 wt %, the precipitation
amount of the γ' phase is insufficient and it is thus difficult to
provide desired high-temperature strength and high-temperature corrosion
resistance. If the content of Al exceeds 6.5 wt %, a large amount of
coarse eutectic γ' phase is formed and solution heat treatment
cannot be performed, and makes difficult to provide desired
high-temperature strength. Accordingly, the content of Al is preferably
4.5 to 6.5 wt % and more preferably 5.0 to 6.0 wt %.

[0047] Ti is an element that enhances the high-temperature strength due to
the actions of solution hardening and precipitation hardening in the
presence of Mo or W. Ti also enhances the high-temperature strength by
the precipitation hardening with relative to the γ'-phase. If the
content of Ti exceeds 1.0 wt %, a harmful phase, such as σ phase
and μ phase, may precipitate to decrease the high-temperature
strength. The content of Ti is therefore preferably 0.0 to 1.0 wt % and
more preferably 0.0 to 0.5 wt %. In the present invention, with a small
amount of Ti or no Ti at all, excellent creep strength in
high-temperature environments may be kept by appropriately determining
the composition ratio of other component elements.

[0048] Hf is an element that segregates at the grain boundary and
distributed unevenly in grain boundary to strengthen the same so as to
enhance the high-temperature strength when the grain boundary
accidentally exists. Furthermore, Hf is excellent in oxidation
resistance, and improves, altogether with Cr and Al, high-temperature
corrosion resistance of the Ni-based single crystal superalloy. If the
content of Hf exceeds 0.5 wt %, local melting occurs to decrease the
high-temperature strength. The content of Hf is therefore preferably 0.00
to 0.5 wt %.

[0049] Nb is an element that enhances the high-temperature strength.
However, if the content of Nb exceeds 3.0 wt %, a harmful phase
precipitates to decrease the high-temperature strength. The content of Nb
is therefore preferably 0.0 to 3.0 wt % and more preferably 0.0 to 1.0 wt
%. With a small amount of Nb or no Nb at all, excellent creep strength in
high-temperature environments may be kept by appropriately determining
the composition ratio of other component elements.

[0050] Re is an element that enhances the high-temperature strength due to
solution strengthening by dissolving in the γ phase which is the
matrix. Re also enhances the corrosion resistance. However, if the
content of Re is less than 3.0 wt %, solution strengthening of the
γ phase becomes insufficient, which makes it difficult to provide
desired high-temperature strength. Here, the present invention is
performed for the Ni-based single crystal superalloy which includes more
Re compared to the conventional alloy, and therefore, the lower limit of
the composition ratio of Re is set to 8.1 wt %.

[0051] If the content of Re exceeds 9.9 wt %, the harmful TCP phase
precipitates in high-temperature environments, which makes it difficult
to provide desired high-temperature strength. The content of Re is
therefore preferably 8.1 to 9.9 wt % and more preferably 8.1 to 9.0 wt %.

[0052] Ru is an element that controls precipitation of the TCP phase to
enhance the high-temperature strength. However, if the content of Ru is
less than 0.5 wt %, the TCP phase precipitates in high-temperature
environments, which makes it difficult to provide desired
high-temperature strength. If the content of Ru exceeds 6.5 wt %, a
harmful phase precipitates to decrease the high-temperature strength. The
content of Ru is therefore preferably 0.5 to 6.5 wt % and more preferably
4.0 to 6.5 wt %.

[0053] The Ni-based single crystal superalloy of the present invention may
further contain for example B, C, Si, Y, La, Ce, V and Zr and the like,
other than incidental impurities. When the Ni-based single crystal
superalloy contains at least one element selected from B, C, Si, Y, La,
Ce, V and Zr, it is preferable that these elements may be included in the
following composition range so as to prevent precipitation of the harmful
phase which might otherwise decrease the high-temperature strength: B:
0.05 wt % or less, C: 0.15 wt % or less, Si: 0.1 wt % or less, Y: 0.1 wt
% or less, La: 0.1 wt % or less, Ce: 0.1 wt % or less, V: 1 wt % or less
and Zr: 0.1 wt % or less.

[0054] On the other hand, Si has an effect which lowers a melting point of
the alloy and may exerts a harmful influence such as local dissolution of
the materials during heat treatment in high-temperature environments such
as solution heat treatment. Therefore, in the Ni-based single crystal
superalloy according to the present invention, it is undesirable to
include elements such as Si and contents of such elements should be
decreased in so far as it is possible.

[0055] As described above, the Ni-based single crystal superalloy
according to the present invention may maintain an excellent creep
strength in high-temperature environments without increasing the specific
gravity whereas it includes a large amount of Re. In the concrete, even
if the content of W is as small as 2.9 wt % or less, or even as small as
1.9 wt % or less, in order to provide a Ni-based single crystal
superalloy having a low specific gravity, excellent creep strength may be
kept in high-temperature environments. Therefore, the Ni-based single
crystal superalloy according to the present invention exhibits excellent
creep strength (i.e., excellent specific creep strength) per unit
density.

[0056] The Ni-based single crystal superalloy according to the present
invention may be used in, for example, a turbine blade 1 as shown in FIG.
1. The turbine blade 1 incorporating the Ni-based single crystal
superalloy according to the present invention has excellent creep
strength in high-temperature environments and may operate for a prolonged
time in high-temperature environments. I addition, the turbine blade 1
has a low specific gravity as compared to the fourth or fifth generation
Ni-based single crystal superalloy. Accordingly, the turbine blade 1 may
be made lightweight and may be operated at higher temperatures.

[0057] Therefore, the Ni-based single crystal superalloy according to the
present invention may be incorporated in, for example, turbine blades
(stator blades and rotor blades) of an aircraft engine, an industrial gas
turbine and other systems. In addition, the Ni-based single crystal
superalloy according to an embodiment of the present invention may also
be applied to components or products to be operated for a long time in
high-temperature environments.

[0058] In the present invention, the composition ratio of the γ
phase and the γ' phase dispersed in the γ phase may be
optimized. The invention may therefore be applied to, for example, an
unidirectional solidified material and a normal casting material with
similar advantageous effects of the present invention, in addition to the
Ni-based single crystal superalloy.

Examples

[0059] Hereinafter, advantageous effects of the present invention will be
described in more detail with reference to Examples. It is to be noted
that the present invention is not limited to the Examples and various
modification may be made without departing from the spirit and scope of
the present invention.

[0060] First, molten metals of various kinds of Ni-based single crystal
superalloys are prepared in a vacuum melting furnace. Alloy ingots of
Examples 1 to 3 of varying compositions are cast from the prepared alloy
molten metals. The composition ratios of the alloy ingots of Examples 1
to 3 are shown in Table 1. Table 1 also shows the composition ratios of
related art Ni-based single crystal superalloys as Reference Examples 1
to 28.

[0061] Next, the alloy ingots shown in Table 1 are subject to solution
heat treatment and aging heat treatment to provide the Ni-based single
crystal superalloys of Examples 1 to 3. In the solution heat treatment,
the temperature is raised stepwise from 1503K-1563K (1230°
C.-1290° C.) to 1573K-1613K (1300° C.-1340° C.) and
kept for 1 to 10 hours or longer. In the aging heat treatment, primary
aging heat treatment is conducted where the ingots are kept at 1273K to
1423K (1000° C. to 1150° C.) for 3 to 5 hours.

[0062] For each of the Ni-based single crystal superalloys of Examples 1
to 3, the condition of the alloy structure is observed with a scanning
electron microscope (SEM). The TCP phase is found in neither of the alloy
microstructures.

[0063] Next, the difference of characteristics regarding Re contents and
specific gravity between the present technology (Examples 1 to 3) and the
prior art (Reference Examples 1 to 28) will be explained with reference
to FIG. 2. In FIG. 2, plotted quadrates denote the Examples 1 to 3 and
plotted rhombuses denote Reference Examples 1 to 28.

[0064] As shown in FIG. 2, the difference of characteristics between the
present technology and the prior art is clearly denoted as a relationship
between the Re contents and specific gravity. In the Ni-based single
crystal superalloys of the prior art, the specific gravity tends to
increase in accordance with an increase of the Re contents. However, in
case of the present technology, an increment (gradient) of the specific
gravity becomes smaller than an increment (gradient) of the specific
gravity of the prior art.

[0065] That is, in the Ni-based single crystal superalloy which includes
8.0 wt % or more of Re which is a heavy metal in its composition ratio,
the specific gravity is inevitably increases.

[0066] However, as shown in FIG. 2, in the present technology, the
composition range of the alloy including Ru which controls TCP phase is
specified to optimal ranges suitable for keeping excellent creep strength
in high-temperature environments. As a result, the Ni-based single
crystal superalloy in which the increment of its specific gravity becomes
smaller than that of the prior art which includes 8.0 wt % or more of Re
is obtained whereas it includes 8.0 wt % or more of Re.

[0067] Next, the Ni-based single crystal superalloys of Examples 1 to 3
are subject to a creep test at the temperature of 1000° C. to
1050° C. and under the stress of 245 MPa. The test is continued
until a creep rupture of the samples, and the duration time is defined as
creep life.

[0068] As shown in FIG. 3, the Ni-based single crystal superalloys of
Examples 1 to 3 have longer creep rupture time as compared to the
high-rhenium Ni-based single crystal superalloy of the Non-Patent
Document 1 which is denoted as Reference Example 1 in FIG. 3.

[0069] Specifically, according to the comparison under the condition of
the above-described creep test, creep rupture time of Examples 1 to 3 are
2007.7 (hrs), 888.4 (hrs) and 828.6 (hrs), respectively, and are longer
than creep rupture time of the high-rhenium Ni-based single crystal
superalloy (593 (hrs)).

[0070] In particular, creep rupture time of Example 1 is more than 3 times
as long as that of the Reference Example 1, and shows extremely superior
creep strength.

[0071] As described above, the Ni-based single crystal superalloy of the
present invention has excellent specific creep strength even though it
includes 8.0 wt % or more of Re.

[0072] Subsequently, the result of simulation performed to compare a
relationship between content of Mo in the Ni-based single crystal
superalloy of the present invention and creep life will be explained with
reference to FIG. 4.

[0073] This simulation is performed by using a software "JMatPro V.5.0"
developed by Sente Software Ltd. (UK). This software calculates values
concerning thermodynamic physical properties and mechanical physical
properties of metallic alloys, and it is demonstrated that the creep life
of the Ni-based single crystal superalloy which is included in the
technical field of the present invention can be accurately estimated as
shown in FIG. 16 of the following document. [0074] (Document: N.
Saunders, Z. Guo, X. Li, A. P. Miodownik and J-Ph. Schille: MODELLING THE
MATERIAL PROPERTIES AND BEHAVIOUR OF Ni-BASED SUPERALLOYS, Superalloys
2004, (TMS, 2004), pp. 849-858.)

[0075]FIG. 4 is a graph obtained by simulation showing a relationship
between content of Mo and stationary creep speed in a Ni-based single
crystal superalloy, and an axis of abscissa denotes content of Mo and an
axis of ordinate denotes stationary creep speed.

[0076] Composition of the alloy used for analysis is set to have an
average composition of Examples 1 to 3 of the present invention and only
the content of Mo is changed to 0.0 to 4.5 wt %, and content of Ni is
regulated in accordance with a change of the content of Mo. In addition,
a condition of analysis is set to 250° C. and 245 MPa on the
assumption of an ordinary situation of a turbine blade in operation.

[0077] From FIG. 4, it is confirmed that the creep speed decreases in
accordance with the increase of the content of Mo, in particular, when
the content of Mo exceeds 2.0 wt % or so, it becomes to show an excellent
creep characteristic (creep speed which is 1/3 or less of that of the
alloy who does not include Mo).

[0078] On the other hand, when excessive Mo is added to the alloy, the
above-described TCP phase is precipitated easier.

[0079] FIG. 5 shows a relationship between content of Mo and precipitation
starting time of TCP phase obtained by simulation.

[0080] Composition of an alloy used for analysis is same as that of the
alloy used for the analysis of FIG. 4, and a temperature for analysis is
set to 950° C.

[0081] From FIG. 5, it is confirmed that the precipitation starting time
of TCP phase is shortened in accordance with the increase of the content
of Mo, in particular, when the content of Mo exceeds 3.0 wt %, it becomes
to fall below 100 hours, and when the content of Mo exceeds 3.5 wt %, it
becomes to fall below 70 hours.

[0082] Accordingly, in order to reduce harmful influences caused by the
precipitation of TCP phase while maintaining excellent creep strength, it
is preferable that the content of Mo is controlled to 2.1 wt % or more to
3.4 wt % or less (more preferably 3.0 wt % or less).

[0083] While preferred embodiments of the present invention have been
described with reference to figures, the present invention is not to be
limited to the above embodiments.

[0084] Each of the features and combinations thereof disclosed in the
above-described embodiments only shows one instance and can be changed
based on design requirements within the spirit or scope of the present
invention.

INDUSTRIAL APPLICABILITY

[0085] According to the present invention, the Ni-based single crystal
superalloy which includes more than 8 wt % of Re in the composition ratio
and has excellent specific creep strength may be provided. Therefore, the
turbine blade incorporating the Ni-based single crystal superalloy which
includes a large amount of Re and has excellent specific creep strength
may be made lightweight and may be operated at higher temperatures.